Update version of PyYAML

Change-Id: Ibaa3e941ac3867c88f24170c28d6e18028546096
Signed-off-by: Mark Beierl <mark.beierl@canonical.com>
(cherry picked from commit a6fc159725f3da99a8ad2847ca1dd14afa26e64a)
diff --git a/requirements.in b/requirements.in
index 9dbfc17..1c47f66 100644
--- a/requirements.in
+++ b/requirements.in
@@ -14,11 +14,10 @@
 # limitations under the License.
 ##
 bitarray
-charm-tools
 haikunator
 pyangbind
 python-openstackclient
-pyyaml<6
+pyyaml>6
 regex
 requests
 robotframework